Question 7.73

image 43. People who eat lots of fruits and vegetables have lower rates of colon cancer than those who eat little of these foods. Fruits and vegetables are rich in antioxidants such as vitamins A, C, and E. Will taking antioxidant pills help prevent colon cancer? A clinical trial studied this question with 864 people who were at risk for colon cancer. The subjects were divided into four groups: those who took daily beta carotene (related to vitamin A), those who took daily vitamins C and E, those who took all three vitamins every day, and those who took a daily placebo. After four years, the researchers were surprised to find no significant difference in colon cancer among the groups.

  1. Outline the design of the experiment. Use your judgment in choosing the group sizes.
  2. Assign labels to the 864 subjects and use Table 7.1 (page 298), starting at line 118, to choose the first five subjects for the “beta carotene” group.
  3. The study was double-blind. What does this mean?
  4. What does “no significant difference” mean in describing the outcome of the study?
  5. Suggest some characteristics of the kind of people who eat lots of fruits and vegetables that might explain lower rates of colon cancer. The experiment suggests that these variables, rather than the antioxidants, may be responsible for the observed benefits of fruits and vegetables.

43.

(a) This is a randomized comparative experiment with four branches. The “flowchart” outline must show random assignment of subjects to groups, the group sizes and treatments, and the response variable (colon cancer). It is best to use groups of equal size, 216 people in each group.

image

(b) With labels 001 to 864, the first five chosen are 731, 253, 304, 470, and 296. In order, these are 253, 296, 304, 470, and 731.

(c) Neither those working with the subjects nor the subjects know the contents of the pill each subject took daily.

(d) The differences in colon cancer cases in the four groups were so small that they could easily be due to the chance assignment of subjects to groups.

(e) People who eat lots of fruits and vegetables may eat less meat or more cereals than

other people. They may drink less alcohol or exercise more.